The present invention pertains to plasma treatment systems suited for effectively carrying out plasma treatment in high-precision manufacturing processes employed in producing integrated circuits (ICs) and liquid crystal displays (LCDs), for example, and, more particularly, relates to plasma treatment systems which produce a plasma by use of electronic and magnetic fields.
Conventionally known examples of plasma treatment systems used in plasma treatment, such as chemical vapor deposition (CVD), etching and ashing, include a so-called parallel plate etcher (RIE) provided with a pair of parallel plates to serve as facing electrodes, in which a plasma treatment space is created between the parallel plates and the plasma treatment performed on a substrate like a silicon wafer, as well as a parallel plate PCVD system used for film-forming operation.
Shown in FIG. 13 is a vertical sectioned constructional diagram, in which a parallel plate type plasma treatment system has a pair of parallel plates provided within a vacuum chamber, and a plasma is produced in or introduced into a plasma treatment space formed between the two parallel plates together with a specific treatment gas, for instance, is also introduced into the plasma treatment space. A plasma-assisted reaction is then produced in the plasma treatment space, whereby an etching process, for instance, is performed on a substrate surface placed within the plasma treatment space.
As an example, an etcher is now described in detail. This system comprises a vacuum chamber fitted with a vacuum chamber cover unit 3 which can be opened and closed. Since a substrate 1, which is a workpiece to be treated, has a flat platelike shape, a horizontally positioned cathode portion 12 is provided approximately in the middle of a main vacuum chamber unit 2, the cathode portion 12 having a flat-shaped top surface covered with an insulating membrane attached thereto so that the substrate 1 can be loaded on the cathode portion 12. A cylindrical lower support 12a is mounted in an upright position in the main vacuum chamber unit 2 passing through a central part of its bottom. The cathode portion 12 is fixed to the top of the lower support 12a and supported thereby. A substrate supporting structure constructed of the aforementioned elements is mounted within the vacuum chamber with its top surface formed into such a shape that allows the substrate 1 to be loaded.
Mounted approximately in the middle of the vacuum chamber cover unit 3 above the cathode portion 12 is an anode portion 11 which is hung by means of a cylindrical upper support 11a. When a high-frequency voltage is applied from an RF power supply 31 to the anode portion 11 and the cathode portion 12, which serve as facing electrodes, a plasma is produced between the anode portion 11 and the cathode portion 12 under specific vacuum pressure. If a specific treatment gas is supplied at this point, plasma treatment is performed on the substrate 1 placed on the top surface of the cathode portion 12 according to the gas state and other conditions. The anode portion 11 thus serves to form a plasma treatment space 13 between itself and the top surface of the cathode portion 12.
Formed in the main vacuum chamber unit 2 by machining is an intake opening 2a passing from the inside to the outside of the main vacuum chamber unit 2 for drawing out internal gases of the vacuum chamber in order to maintain a proper degree of vacuum. A gate valve 4a, a variable valve 4 and a vacuum pump 5 are connected to the intake opening 2a in this order. The gate valve 4a is a manually-operated valve for blocking gas flow during maintenance, for instance, and is kept in an open position during normal operation. The variable valve 4 connected between the gate valve 4a and the vacuum pump 5, such as a turbopump, is associated with a motor, for instance, which can variably change the valve opening. The motor is controlled by an electric signal so that the variable valve 4 works as a variable throttle which can remotely be controlled to regulate flow of a fluid. The vacuum pressure inside the vacuum chamber is measured by a vacuum gage 4b fitted to the vacuum chamber. When a control signal is generated by a PID control circuit 4c based on the difference between a measurement value of the vacuum gage 4b and a predefined target value, throttle setting of the variable valve 4 is varied in accordance with the control signal. The vacuum pressure within the vacuum chamber is automatically controlled by pressure control means having the vacuum gage 4b as a pressure sensor, the PID control circuit 4c as a pressure control circuit and the variable valve 4 as a pressure control mechanism as described above.
The density of plasma is insufficient in the above-described example for plasma generation, in which an electric field is applied across the parallel plates. Another known example is an arrangement in which a high-density plasma (HDP) is produced by additionally applying a magnetic field to confine the plasma. This arrangement is employed in an MRIE (magnetron reactive ion etcher), for example, in which the ratio of ion species in the composition of plasma is increased as the plasma density becomes higher. Besides the fact that the plasma tends to be unevenly distributed in this type of arrangement, the arrangement shows a tendency to cause severer damage by ions to the workpiece to be treated when the ratio of ions is increased. There exists a system devised to prevent such damage by producing a uniform magnetic field through the use of a flat-shaped coil as described in Japanese Unexamined Patent Publication No. 3-79025. The workpiece to be treated is still exposed directly to a high-density plasma being produced in this method, however, although the Publication contains no mention of charge-up problem of the workpiece to be treated caused by a plasma current and other problems arising from direct exposure to the plasma.
On the other hand, also known in the art are plasma etching systems in which an entire plasma space is divided into separate plasma spaces, that is, a plasma treatment space and a plasma producing space which are connected to each other, in order to reduce damage to a workpiece to be treated caused by ions and to prevent its direct exposure to a high-density plasma being produced, wherein the ratio of radical species is increased by reducing ion species in the composition of the plasma when the high-density plasma produced in the plasma producing space is delivered to the plasma treatment space. The systems of this type are classified into several alternatives including such systems as an ECR (electron cyclotron resonance) system using radical flow and those described in Japanese Unexamined Patent Publication No. 4-81324 in which the two spaces are separated by a great distance, such systems as an ICP (inductive-coupled plasma) system in which a high-density plasma is confined within a plasma producing space located adjacent to a plasma treatment space by means of a powerful magnetic field, and such systems as described in Japanese Unexamined Patent Publication No. 4-290428 in which a high-density plasma is confined by using circularly polarized electromagnetic waves emitted from a ring antenna although this alternative is same as the preceding alternative in that a plasma producing space is located adjacent to a plasma treatment space.
Among these conventional plasma treatment systems, however, the aforementioned ECR type of treatment system in which the two spaces are separated by a great distance does not provide so much an improvement in plasma treatment efficiency as might be expected from the ratio by which the amount of radical species is increased by reducing the ratio of ion species by more than a necessary level. This is because there exist many restrictions on the manner of mounting those mechanisms which make it possible to separate the plasma treatment space and the plasma producing space by a proper distance from each other. Previous attempts to develop a plasma treatment system based on the ECR type capable of providing good treatment efficiency at a proper plasma constituent ratio have so far been unsuccessful, because even if the ratio of radical species constituents to ion species constituents in the plasma is brought close to a level where a high plasma treatment efficiency is achieved by devising an improved method of mechanism mounting, the desirable ratio itself varies when the type or pressure of active gas, or the material of the workpiece to be treated is changed, and also because it is difficult to realize a mechanism which makes it possible to controllably vary the distance between the two spaces.
In the ICP type, ionization occurs and a plasma is produced and formed when electrons are accelerated as a result of magnetic field variations caused by variations with tine of a current flowing through a coil and their energy exceeds a level which causes a surrounding treatment gas to ionize. The high-energy electrons useful for the ionization are produced in a doughnut shape since an ionization mechanism is formed in a focused state depending on a combined magnetic field created by the coil. Since electron energy distribution approximately follows the Boltzmann distribution, electrons having energy levels equal to or higher than the ionization level cause ionization of gases within the plasma space whereas electrons having energy levels less than the ionization level produce radicals. Thus, it is not possible to arbitrarily set and control the density ratio of ions and radicals in the ICP-type plasma because formation of the ions and radicals depends on the same plasma producing means as described above. A TCP plasma (transformed-coupled plasma) system also employs roughly the same mechanism although the shape of its coil is different.
On the other hand, the approach using the circularly polarized electromagnetic waves makes it possible to avoid the use of a powerful magnetic field. In this approach, however, the plasma producing space has an expanse roughly comparable to the plasma treatment space, or an expanse at least equal to or larger than the workpiece to be treated, to ensure the uniformity of plasma distribution within the plasma treatment space, because a single ring antenna having a large diameter is employed. Thus, the two spaces are connected to each other at their adjoining surface which also has a large expanse.
If a connecting part between the two spaces has a large cross-sectional area, the amount of gases flowing in a reverse direction from the plasma treatment space into the plasma producing spaces increases. This problem is common to all such conventional plasma treatment systems that employ the separated but adjacent plasma treatment and plasma producing spaces. This also applies to the ECR type to nearly the same extent. Although the plasma treatment space and the plasma producing space of this type would appear as being separated from each other at a first glance as they are physically spaced apart unlike the TCP and ICP plasma types, they are actually not so distinctly separated as they would seemingly be expected as long as the plasma constituents are concerned, because an opening of their connecting part has a large diameter.
The aforementioned counterflow gases contain such constituents that have occurred as a result of treatment of the workpiece and should be discharged as quickly as possible although their quantities are rather small. Since these gases to be discharged are severely decomposed and ionized by the high-density plasma when they enter the plasma producing space, they could be changed in properties, forming undesirable substances which would impede proper treatment or cause contamination of the interior of the system in many cases. Even though the two spaces are seemingly divided, they are not so distinctly separated from each other as long as the plasma constituents are concerned.
Thus, it is difficult to provide high-quality treatment unless the counterflow of the undesirable gases can not be prevented even when the uniformity of plasma distribution can be maintained.
It might be possible to decrease the amount of gas passage by fitting a baffle plate at the adjoining surface through which the two spaces are connected to each other to reduce the cross-sectional area of the connecting part. Although the amount of inflow gases will decrease, the amount of outflow gases will also decrease in this case. As a consequence, gases which have once entered the plasma producing space would not easily come out and, therefore, the ratio of gases whose properties are altered by the high-density plasma would be increased. For this reason, the prevention of change in gas properties, an ultimately desired effect, is not likely to be achieved even when the arrangement as described in Japanese Unexamined Patent Publication No. 4-290428, for instance, is combined with the baffle plate, for instance.
Thus, one problem to be solved is to devise a construction of the two spaces which can effectively prevent gas flow from the plasma treatment space to the plasma producing space.
It is, however, desired to hold such prerequisites that the entire plasma space be separated into a plasma treatment space and a plasma producing space for reducing plasma damage and charge-up and the plasma producing space and the plasma treatment space be located adjacent to each other in order to achieve a proper ratio between radical species constituents and ion species constituents within the plasma.
With an increase in the physical size of substrates to be subjected to the plasma treatment, it has become common to employ a single-substrate processing method in plasma treatment systems, in which each successive substrate is treated alone at one time. Under this circumstance, the top surface of one of a pair of parallel plates that serves as a substrate supporting structure is almost entirely covered with a substrate and, therefore, it is difficult to locate an intake opening in the middle of the substrate supporting structure in a previously used fashion. For this reason, the intake opening is formed outside the location where the substrate supporting structure is mounted in an upright position in the main vacuum chamber unit. As the size of the substrate increased, the parallel plates as well as the internal volumetric capacity of the chamber and the intake opening are also becoming larger.
Notwhithstanding the increase in the substrate size, requirements for accuracy and uniformity of treatment have never been relaxed, but are becoming even more stringent. It has then become necessary to maintain the uniformity of the state of plasma throughout the whole top surface area of the substrate to meet these requirements. If, however, the intake opening is offset from the center of a target, flow of plasma and other substances will deflect, making it difficult to maintain the uniformity of the plasma. Even when intake openings are dispersed in symmetrically arranged positions, the same problem will occur if there is a difference in the length of piping between the individual intake openings and a vacuum pump. Although it might be possible to install a baffle plate in from of each intake opening in the vacuum chamber to produce a uniform flow on the upstream side of the baffle plate (see 2b in FIG. 13) to cope with the problem, this arrangement would entail an increase of the internal volumetric capacity of the chamber and the baffle plate would work as a resistance to the flow, thus causing a loss of pressure controllability. Specifically, it would become impossible to swiftly control pressure condition of the plasma, causing increased pressure variations, and thereby making it difficult to maintain the plasma pressure at a desired setting.
On the other hand, it can be said that it is effective in increasing the plasma pressure controllability to reduce the capacity of the vacuum chamber. One specific approach would be to reduce the size of the vacuum chamber down to such a point that the inside wall surface of the chamber would scarcely come into contact with outer edges of the parallel plates (see FIG. 14). If the capacity of the vacuum chamber is reduced simply by using such forcible approach, however, there is no way but to locate the intake opening right at the side of the plasma treatment space. Thus, this approach could produce a severely deflected plasma flow, causing a significant deterioration of the uniformity of the plasma. Since there is not any spatial room for installing a baffle plate in this case, it would become difficult to install a substrate input and output mechanism, for instance, and maintenance work of the rear sides of the parallel plates, for instance, would also become difficult.
Besides the uniformity of plasma treatment, capability to cope with the tendency toward large-sized substrate is also an important prerequisite for the plasma treatment systems and, thus, it is not possible to maintain and enhance the value of these systems as manufactured products if they are barely good enough to meet only one of these requirements.
Accordingly, another problem to be solved is to devise a construction which makes it possible to control the pressure condition of the plasma, for instance, to create an even more uniform state even more swiftly, in order to meet the aforementioned contradictory requirements.